I know that you can trade Pokemon between games of the same generation (Gold/Silver, Ruby/Sapphire) and even trade between 1st gen and 2nd gen. However, I haven't been able to find a way to trade Pokemon so that I can play with my 1st generation Pokemon in HeartGold, for instance. If it is possible, how many steps would it take and what equipment would I need?

There are three main reasons you can't trade between Gen I/II and Gen III:

The Pokémon data format changed significantly in Gen III, so the two are incompatible.

The GBA link cable uses a different voltage to the Gameboy link cable. Even though a GBA can play old Gameboy games, it uses different circuitry than when playing GBA games, so the games' link cable functionalities are fundamentally incompatible.

Even if these weren't an issue, Nintendo never released a product that allowed trade from one to the next.

*(Trading with Pokémon Bank is supposed to work with all future Generations, although no specifics are known)

Excellent answer, but #2 on your list is not entirely true: it's definitely possible to connect up a GBA and original Gameboy, I've done it. You need a special cable though.
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DisgruntledGoatDec 16 '10 at 23:47

6

If I recall correctly, you can link a Gameboy to a GBA, but only with Gameboy games. The cable operates either either Gameboy-mode or GBA-mode, but never both at once. Hence it's impossible to trade between generations, even if the game's software supported it.
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Jonathan DrainDec 17 '10 at 11:58

I ve heard you could put 2gen pokemon on Pokemon Stadium, and get them back to 3gen, but never had a nintendo 64. Anyone could confirm?
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DrakaSANApr 11 '14 at 13:02

@DrakaSAN: The Pokémon Stadium games predate 3rd generation by about 2 years, and you can't fit a GBA cartridge in a Transfer Pak anyway - so no, that's not possible. There's no path between 2nd and 3rd generation.
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Michael MadsenApr 16 '14 at 22:31

While it may not be possible to transfer from Gens I and II to III, seeing as Gen III has FireRed and LeafGreen, it would be advisable to just play through that and trade upwards (to gens IV and V) from there..

*It is not possible to trade pokémon from generation 5 to generation 6. As explained on the official website: "There are a number of differences between the functions and specifications of the Nintendo 3DS family of systems compared to other systems in the Nintendo DS family (DSi XL/DSi/DS Lite/DS), meaning that it is unfortunately not possible to communicate directly between software made for the two different systems (models)".

However, a paid service called Pokémon Bank has been created to fulfil this gap. The service costs $4.99 (yearly subscription) and it allows users to upload their pokémon to the cloud. The "uploading" service can be used on both 5th and 6th generations, while the "downloading" service can only be used on Pokémon X and Y. Using this service allows de facto the users to trade pokémon from generation 3 up to 6.

You could have just edited the accepted answer to include this new information instead of creating a new answer and basically copy most of that answer.
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sceniaApr 11 '14 at 12:19

Hi @scenia, I am afraid I didn't know I could edit other users answers. Also, it seems quite invasive as approach, but I may be wrong. I only wanted to update the information here presented, that's all.
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Annoys ParrotApr 12 '14 at 13:37

1

It's not invasive, that's how this site works ;)
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sceniaApr 12 '14 at 15:05